Surface processing such as surface hardening or shot-peening by heat treatment, nitriding, and the like is applied to steel parts such as gears and shafts used as automobile parts, etc. to improve friction resistance, fatigue strength, and the like.
Conventionally, evaluation of surface properties such as residual stress and hardness after surface processing of such products has been performed by sample destructive inspection. This resulted in the problem that not all products could be directly inspected, and inspected products became unusable, since inspection was destructive.
This has led to a strong requirement to develop a device capable of non-destructively inspecting product surface properties. As an example of such a device, Patent Document 1 discloses a non-destructive inspection apparatus for a shot-peened surface, wherein an AC signal is input while varying the frequency to an inspection circuit furnished with a coil disposed above a shot-peened surface, and the state of generation of residual stress in the inspected object is inspected using the impedance frequency response characteristics of the inspection circuit.